From: Phrogz <phrogz@...>
Date: 2007-09-22T12:05:07+09:00
Subject: Re: foo || 0 will cause exception but not for himself

On Sep 21, 8:37 pm, SpringFlowers AutumnMoon
<summercooln...@gmail.com> wrote:
> if foo is undefined, it will throw exception when referred to... as in
> try 1, 2, and 3 below.  However, it won't throw exception when assigned
> to himself...  how is that interpreted?
>
> irb(main):001:0> foo
> NameError: undefined local variable or method `foo' for main:Object
>         from (irb):1:in `Kernel#binding'
>
> irb(main):002:0> foo || 0
> NameError: undefined local variable or method `foo' for main:Object
>         from (irb):2:in `Kernel#binding'
>
> irb(main):003:0> bar = foo || 0
> NameError: undefined local variable or method `foo' for main:Object
>         from (irb):3:in `Kernel#binding'
>
> irb(main):004:0> foo = foo || 0
> => 0
>
> irb(main):005:0> foo
> => 0

When Ruby says "undefined local variable or method", what it means is
"Hey, I don't know what this is, and I don't know if it's supposed to
be a variable or a method."

When you write "foo = ..." Ruby says, "Ah ha! Foo is supposed to be a
variable. I'll treat it as one."

That's my (somewhat informed) guess anyhow. The determination of the
method/variableness of 'foo' is determined before runtime, when the
lexer/parser is analyzing the program. This is similar to why this
code...
  if foo=42
    p foo
  end
works just fine, but this code:
  p foo if foo=42
throws an error. When it first see's foo in the first code sample, it
knows that it is a variable. When it first sees foo in the second code
sample, it doesn't know what it is yet, and throws an error to
indicate its confusion.